Field of the Invention
The present invention relates to an objective optical system which has a focusing function, and particularly, to an objective lens for endoscope which enables a close observation, and to a taking lens of other small-size cameras for consumer use.
Description of the Related Art
A normal objective lens for endoscope has a depth of field of a wide range from about 5 mm to 100 mm on an object side. In an endoscope equipped with such objective lens for endoscope, an image is provided mainly by using a solid image pickup element such as a CCD (charge coupled device).
In recent years, for improving an accuracy of diagnosis, an improvement in a quality of an endoscope image has been sought. In order to fulfil such requirement, in an endoscope, increasing the number of pixels of CCD, or in other words, making the number of pixels of CCD large has been progressing. However, in an endoscope in which a CCD with a large number of pixels is used, the depth of field becomes narrow as compared to that in a conventional endoscope. Main reasons for the narrowing of the depth of field are that there is a need to make small an Fno (F-number) of an objective lens for endoscope in order to avoid degradation of image quality due to diffraction of light, and that it is necessary to make a focal length of the objective lens for endoscope long as the CCD becomes large due to an effect of making the number of pixels large.
In this case, for securing the depth of field of a same level as in the conventional endoscope, a focusing function is to be imparted to the objective lens for endoscope. For such reasons, there is an increased need of objective lenses for endoscope having the focusing function.
It is desired that the objective lens for endoscope having the focusing function can be used similarly as the conventional objective lens for endoscope. Therefore, in the objective lens for endoscope having the focusing function, it is required that an angle of view of observation does not change when focused.
As an objective lens having the focusing function and a small fluctuation in the angle of view, objective lenses (objective optical systems) disclosed in Japanese Patent Publication after Examination No. Sho 55-015005, Japanese Patent Application Laid-open Publication No. 2000-330015, Japanese Patent Application Laid-open Publication No. 2002-28126, and Japanese Patent Publication No. 4819969 are available. The objective lens in Japanese Patent Publication after Examination No. Sho 55-015005 includes two units having a refractive power in order of a negative refractive power and a positive refractive power, and the objective lens in Japanese Patent Publication No. 4819969 includes three units having a refractive power in order of a negative refractive power, a positive refractive power and a positive refractive power. In both the objective lenses, the focusing is carried out by moving a second unit. Moreover, in Japanese Patent Application Laid-open Publication No. 2000-330015 and Japanese Patent Application Laid-open Publication No. 2002-28126, an objective lens including two units having a refractive power in order of a positive refractive power and a positive refractive power, has been disclosed.
In addition, as an objective lens for magnifying endoscope which enables to carryout focusing to an object point at a closer distance, objective lenses disclosed in Japanese Patent Publication after Examination No. Sho 61-044283, Japanese Patent Application Laid-open Publication No. Hei 06-317744, and Japanese Patent Application Laid-open Publication No. Hei 11-316339 are available. These objective lenses for magnifying endoscope includes three units having a refractive power in order of a positive refractive power, a negative refractive power, and a positive refractive power, and the focusing is carried out by moving a second unit having a negative refractive power. Moreover, in Japanese Patent Application Laid-open Publication No. 2000-267002, an optical system which includes three units having a refractive power in order of a negative refractive power, a positive refractive power, and a negative refractive power, and in which the focusing is carried out by moving a second unit having a positive refractive power, has been disclosed.